The emergence and widespread application of various schemes for wireless and satellite communications has prompted research on low cost candidates for components of a communication system. The simplicity of the manufacturing process, reliability and ease of operation are among the other driving factors in the design of wireless systems. Antennas as the radiating and receiving elements are not an exception to this general trend. Recently, planar reflectors have been considered as a viable option that fulfils the stringent design requirements of wireless systems. Such a planar array is described in D. C. Chan and M. C. Huang, "Microstrip reflectarry with offset feed", Electronics Letters, pp. 1489-1491, July 1992. Ease of manufacturing, deployment and operation are among the advantages of planar array antennas. More importantly, planar reflectors tend to minimize the feedline losses and thus enhance the effective utility of printed structures.
The physical principles governing the operation of planar printed reflectors are discussed previously in D. M. Pozar and T. A. Metzler, "Analysis of reflectarray antenna using microstrip patches of variable sizes", Electronics Letters, pp. 657-658, April 1993, and in F. S. Johansson, "A new planar grating reflector antenna", IEEE Trans. Antenna and Propagt, Vol. 38, No. 9, pp. 1491-1495, Sept. 1990. In general, an electromagnetic wave impinges on the surface of a planar reflector whose elements were designed so as to change the phase front of the electromagnetic excitation. Various methods such as dimensioning the patches, loading the patches, or proper placement of patches were used as means of transforming the phase front of the incoming wave. The fact that all of these methods are frequency dependent, has made planar printed reflectors prone to a beam squint as the frequency is scanned within the band.
A planar reflector with an offset feed is often designed to provide a high gain antenna, producing a collimated reflected signal. Since gain is related to beam width, a narrower more collimated beam is often desired. Unfortunately, as the distance between a transmitter and receiver is increased, a collimated beam must be more accurately armed from the transmitter in order to reach the receiver. When a collimated beam shifts a few degrees, the receiver may not even receive the outer edges of the beam. Also, as the beam direction changes, the receiver becomes more or less centrally located within the beam. This affects signal levels and therefore, affects signal to noise ratios. As such, it is important to direct a beam accurately from a transmitter.
Beam squint effectively alters an angle of reflection of a signal from a planar array. In essence, as the frequency of the signal varies, the angle of reflection of the signal also varies. This inherent limitation of planar printed reflectors is well known in the art and severely restricts application of planar reflector array antennas in satellite communications. Because of the close proximity of adjacent satellites in space, beam squint implies reception of unwanted signals from neighbouring satellites.
For communications, different applications are commonly allotted a set of frequencies--a frequency band. It would be advantageous to provide a planar reflector array antenna that has reduced beam squint over such a frequency band.